This patch adds support for controlling a playback operation from the

Asterisk REST interface.

This adds the /playback/{playbackId}/control resource, which may be
POSTed to to pause, unpause, reverse, forward or restart the media
playback.

Attempts to control a playback that is not currently playing will
either return a 404 Not Found (because the playback object no longer
exists) or a 409 Conflict (because the playback object is still in the
queue to be played).

This patch also adds skipms and offsetms parameters to the
/channels/{channelId}/play resource.

(closes issue ASTERISK-21587)
Review: https://reviewboard.asterisk.org/r/2559


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@389603 65c4cc65-6c06-0410-ace0-fbb531ad65f3